Here some reason to upgrade:

PSR S970 PSR S950
Large 7 inch screen vs 5.2
Flash memory 8 times(512 Mgs) than the 64 Mb
40 Audio Styles vs 25
More styles 450 vs 408
Same DSP as Tyros5(VCM) vs previous gen
Real Choir Voice Vs more synthetic choir

The New DSP (Digital Signal Processors or Effects), make a Big difference, makes everything sound so much better.

Manuel